This fixes a rare bug in polygon stippling with non-monolithic pixel shaders.

The underlying problem is as follows: the prolog part contains the polygon
stippling sequence, i.e. a kill. The main part then enables WQM based on the
_reduced_ exec mask, effectively undoing most of the polygon stippling.

Since we cannot know whether polygon stippling will be used, the main part
of a non-monolithic shader must always return to exact mode to fix this
problem.

Re-thinking the approach: I think it's best to just say that SI_RETURN
requires a switch to exact, since SI_RETURN is what marks the return in
non-void returning shaders. Also added an unreachable test.